Human inhalation of chlorine and other toxic gasses leads to several acute and chronic respiratory abnormalities including Acute Lung Injury (ALI), Reactive Airways Dysfunction Syndrome (RADS), and Airway Intraluminal Fibrosis (AIF). There is currently no known therapy to prevent the development of these disorders after chemical inhalation. In the case of AIF, there is also a fundamental gap in understanding how intraluminal fibrotic lesions are stimulated and develop. Our objective in this renewal application is to develop medical counter-measures that will prevent the development of ALI, RADS, and AIF when administered after chlorine exposure. The central hypotheses of this proposal are that 1) chemical-induced ALI and RADS can be prevented by post-exposure administration of the iNOS inhibitor GW274150; 2) chemical-induced AIF can be prevented by inhibiting specific receptor tyrosine kinase signaling pathways; and 3) post-exposure airway epithelial repair can be enhanced by stimulating the survival and proliferation of airway epithelial stem cells with existin drugs. These hypotheses are based on strong preliminary data generated during the 3 years of a previous U01 award and will be tested in three specific aims: 1) to develop GW274150 as a human therapeutic for chlorine induced ALI and RADS, 2) to identify specific tyrosine kinase inhibitors that prevent the mesenchymal cell proliferation, angiogenesis, and fibrosis that lead to AIF, and 3) to identify existing drugs that promote the survival and proliferation of epithelial stm cells. In the first aim, GW274150, a specific iNOS inhibitor that has already been tested in phase III human clinical trials for other indications, will be examined in murine and pilot large animal efficacy stuidies of chlorine-induced ALI and RADS. If proven efficacious, GW274150 will be subjected to additional studies necessary for regulatory approval, including collection of pharmacokinetic and toxicity data, production of GW274150 using current good manufacturing practices, and definitive large animal efficacy studies. Once completed, approval of GW274150 for human use will be sought under the FDA animal rule. In the second aim, the strategy of inhibiting specific tyrosine kinase signaling pathways, which has already been validated in proof-of-principle studies, will be translated to clinical utility by determining the efficacy of specifi tyrosine kinase inhibitors in a novel murine model of chlorine-induced AIF. In the third aim, candidate therapeutics that stimulate epithelial stem cell proliferation and differentiation will b identified in a novel in vitro screen of small molecules and FDA approved drugs. Two compounds already identified, along with additional hits, will be validated and tested for efficacy in a murine model of epithelial cell repair. The successful completion of the proposed studies will result in the first FDA-approved therapy for chlorine-induced ALI and RADS, preclinical validation of the first therapy for chlorine-induced AIF, and the identification of existing drugs that stimulate epithelial regeneration after chemical inhalation.